1. Field of the Invention
The present invention relates to a small diameter stepping motor including two stator units each including a bobbin, and to a method for manufacturing the stepping motor.
2. Description of the Related Art
Stepping motors are extensively used in various electronic devices. Some stepping motors include two stator units each including a bobbin which is made of an insulating material, and which has a circular hollow cylinder portion, two flanges formed integrally with the cylinder portion so as to be disposed respectively at the both ends of the cylinder portion, and a terminal structure disposed at one flange of the two flanges and adapted to terminate a pair of lead-out lines of a winding disposed around the cylinder portion. The terminal structure generally includes a terminal block made of an insulating material integrally with the one flange, and two electrical contact members (for example, terminal pins) implanted in the terminal block and connected to the pair of lead-out lines of the winding.
A stepping motor for use in small devices, such as a digital camera (refer to FIG. 11 showing, as an application example, a mechanism of a typical digital camera, where a stepping motor 1 having a small diameter is incorporated in a shutter mechanism 2 disposed behind a lens of a digital camera, such that the stepping motor is disposed at the outer circumference of a circular cylindrical frame 3 to support the shutter mechanism 2, and is adapted to drive the lens for automatic focusing, to conduct an aperture mechanism, and to perform other like functions.) and a mobile telephone, is increasingly required to be downsized without impairing its performance or even with achieving an enhanced performance. Under such circumstances, the configuration of the terminal structure of a bobbin is a critical factor in terms of ensuring the workability of assembling a stepping motor in the effort of downsizing its diameter.
FIGS. 12 to 15 each show both or one of a pair of conventional bobbins 101 and 201 to be housed in respective stator frames. The bobbins 101 and 201 are structured to achieve the downsizing of a stepping motor (refer to Japanese Patent Application Laid-Open No. 2004-7899).
Referring to FIGS. 12 and 13, the bobbins 101 and 201 are made of an insulating material, and each of the bobbins 101 and 201 includes a circular hollow cylinder portion 102 (202), and two flanges 103 and 103 (203 and 203) integrally formed with the cylinder portion 102 (202) and disposed respectively at the both ends of the cylinder portion 102 (202). The bobbins 101 and 201 structured as described above and housed in respective stator frames (not shown) are put together coaxially with respect to an axis O such that their respective one flanges 103 and 203 are set adjacent to each other.
The bobbins 101 and 201 further include terminal structures 111 and 211, respectively. The terminal structures 111 and 211 are disposed respectively at the one flanges (hereinafter referred to as terminal flanges as appropriate) 103 and 203 which are set adjacent to with each other. The terminal structure 111 (211) includes a terminal block 121 (221), a bridge portion 122 (222) connecting the terminal block 121 (221) integrally with the terminal flange 103 (203), and a pair of terminal pins 131 (231) and 132 (232) as electrical contact members which are made of an electrical conductive material, implanted at the terminal block 121 and to which lead-out lines of a winding wound around the cylinder portion 102 (202) are connected.
The bridge portion 122 (222) is formed integrally with the terminal flange 103 (203) so as to extend axially outwardly from the outer circumference of the terminal flange 103 (203), that is to say, extend away from the other flange (non-terminal flange) 103 (203). The terminal block 121 (221) integrally extends axially from the distal end of the bridge portion 122 (222) so as to overhang a part of the cylinder portion 202 (102) of the bobbin 201 (101) to which the bobbin 101 (201) is coupled.
Referring additionally to FIGS. 14 and 15, the bridge portion 122 has a width (circumferential direction dimension) defined by W1, and is positioned circumferentially off a diametrical line L1 (FIG. 14) of the bobbin 101, which passes substantially the center of the terminal block 121. In the same way, though not specifically shown, the bridge portion 222 has a width (circumferential direction dimension) defined by W1, and is positioned circumferentially off a diametrical line (not shown) of the bobbin 201, which passes substantially the circumferential center of the terminal block 221.
As seen in FIG. 14 and understood from the preceding description, the terminal block 121/221 is divided into two substantially equal portions with respect to the diametrical line L1 of the bobbin 101 (201), and the terminal pin 131 (231) disposed at one half portion of the terminal block 121 (221) is located line-symmetric with the terminal pin 132 (232) disposed at the other half portion thereof with respect to the diametrical line L1.
The above-described two bobbins 101 and 201 having their respective windings thereon are coupled to each other in the following manner. The bobbins 101 and 201 are brought together coaxially such that the respective terminal flanges 103 and 203 having the terminal structures 111 and 211 are set adjacent to each other with their bridge portions 122 and 222 circumferentially shifted from each other. Then, the bobbins 101 and 201 are rotated relatively with respect to each other in the circumferential direction indicated by an arrow Z (see FIG. 14) until the respective opposite circumferential ends of the bridge portions 122 and 222 come into contact with each other. The bobbins 101 and 201 arranged as described above are disposed in the respective stator frames (not shown).
The above-described configuration of the terminal structure 111 (211) of the bobbin 101 (201) is advantageous and effective in reducing the diameter of a stepping motor, and the following description explains the background for the advantage and effectiveness by taking as an example a stepping motor having a bobbin diameter of, for example, more than 10 mm, which is generally considered to be a relatively large diameter in the field of the present invention, specifically for use in a digital camera, a mobile telephone, and the like.
Referring to FIGS. 16A and 16B showing typical terminal pin layouts of stepping motors with a relatively large diameter as described above, in the both layouts, when two bobbins 301 and 401 are coupled to each other, a pair of terminal pins 331 and 332 of the bobbin 301 and a pair of terminal pins 431 and 432 of the bobbin 401 are usually arranged in a substantially straight line located along the interface plane between two stator frames housing the bobbins 301 and 401, respectively. In such a terminal pin arrangement, a distance S1 defined between the respective centers of two adjacent terminal pins (for example, terminal pins 331 and 431) is about half of a distance S2 defined between the respective centers of the pair of terminal pins 331 and 332 (431 and 432) of the bobbin 301 (401). Under the circumstance described above, if the stepping motor is to be downsized making the distance S1 smaller, it becomes practically difficult to perform a soldering operation for connecting the terminal pins 331, 332, 431 and 432 to a flexible printed circuit (FPC), and the like.
Accordingly, if the minimum distance required between two adjacent terminal pins for allowing a soldering operation to be performed at each terminal pin without impairing the workability is defined “S3 (not indicated in FIGS. 16A and 16B; to be further described later with reference to FIG. 5C)”, the aforementioned distance S2 between the centers of the pair of terminal pins must be set equal to or greater than double the minimum distance S3, thus establishing a formula: S2≧2×S3 (or S2/2≧S3). Since the distance S2 is proportional to the diameter of a bobbin, the minimum value of the bobbin diameter is subject to restriction, and it is practically not possible to set the diameter of a stepping motor at, for example, 10 mm or less.
On the other hand, in the pin terminal arrangement resulting from the structure of the bobbin 101 (201) shown in FIGS. 12 to 15, it is not necessary to pay attention to the above-defined distance S1 in consideration of the workability in a soldering operation, and the above-defined distance S2 can be as small as the above-defined distance S3, thus establishing a formula: S2≧S3. Consequently, the diameter of the bobbin 101 (201), to which the distance 2S is proportional, can be reduced to about half of the diameter of the bobbin 301 (401) whose terminal pin layouts are shown in FIGS. 16A and 16B.
In the bobbin 101 (201), however, the bridge portion 122 (222) formed by injection molding, or a like method integrally with the terminal flange 103 (203) is likely to be deformed as a whole causing positional change of the terminal block 121 (221) formed to communicate directly with the bridge portion 122 (222), which results in materially deteriorating the positional accuracy of the pair of terminal pins 131 and 132 (231 and 232) implanted in the terminal block 121 (221), thus yielding defective windings and also hindering automation of a soldering process at an FPC.
Also, when the bobbins 101 and 201 are coupled to each other, the terminal flanges 103 and 203 having their respective terminal structures 111 and 211 are coaxially set adjacent to each other by axial direction operation with their respective terminal structures 111 and 211 shifted from each other, and then the bridge portions 122 and 222 of the terminal portions 111 and 211 are brought into contact with each other by circumferential rotation operation, thus requiring two different operation steps, which results in an increased time for production.
Further, the lead-out lines from the windings, if loosely wired to the terminal pins 131, 132, 231 and 232, are likely to be caught by some portions of the bobbins 101 and 201 and broken at the time of the aforementioned rotation operation.